Just to let you know how things are in my mixed FIV+ and FIV- household.
I rescued a stray who had an appalling limp, shredded ears, no manners whatsoever, but had been feeding at my house for months and got along nicely with my other cats. I trapped him and took him to the local vet who neutered him under the RSPCA voucher scheme for me.
Chubby, as we had named him, was blood tested as my household at the time was all FIV-, so I wanted to know if Chubby could come and live with us, thinking that FIV+ would be dangerous. I was gutted when the tests came back positive (we double checked with the lab in Glasgow) but the vet summoned me in for a chat.
My vet said this: many domestic cats are FIV+ without their owners even knowing. Most never succumb to it and the only statistics available are from the stray cat population that only ever get tested for FIV+ when they are rescued, and even these high risk cats only come in at 40% positive. No-one keeps stats of how many domestic cats are FIV+ the proportion could very well be exactly the same.
As Chubby was already on good terms with my residents (three of whom are rescued strays but never blood tested so may indeed be +ve themselves) we decided to give him a chance so he came home with me rather than going to the CPL. I kept him in until his stink subsided and cat proofed the garden so he could go out into the sunshine but not escape (he can't climb because of his dicky hips).
One year on, he never fought, clawed or bit any of my resident cats, he's a sweet boy who has befriended my only female cat and they are now inseparable, and although he still won't allow me to pet him I'm so pleased we have helped this little guy out.
I hope this gives you some hope, FIV+ is not a death sentence and PTS is totally unnecessary unless the animal has secondary symptoms.
I still have views about FeLV, though, as this is highly contagious, unlike FIV+ which is usually only transmitted by deep bite wound, sexual contact or blood transference. I understand that FeLV is also tested for, but you would be wise to ask if this has in fact been done. Bless you for caring.